The present invention relates to an ion beam accelerating device for providing energy to charged particles, and in particular, to an ion beam accelerating device which is suitable for application to a medical use or physical experiments.
First of all, an accelerating cavity to be used for accelerating ion beams will be described in the following. Because a proton which has the lightest mass of ions is about 2000 times heavier than an electron, the relativistic effect of ions is small. Therefore, the velocity of an ion is generally slow, and in addition, ion velocity undergoes a substantial change during acceleration. Thereby, in order to accelerate an ion beam to a predetermined energy level, a magnetic core-loaded accelerating cavity in which magnetic cores are installed is used by advantageously decreasing its resonant frequency in accordance with the magnetic permeability of the loaded magnetic cores. There are two types of this magnetic core-loaded accelerating cavity: one is a tuned-type accelerating cavity, which uses a magnetic core having a low magnetic loss and controls the magnetic permeability of the magnetic core by applying a bias magnetic field using a bias current, so that the magnetic permeability thereof is tuned to the resonant frequency; and the other is an untuned-type accelerating cavity, which actively makes use of magnetic loss and can broaden the resonance frequency band, although its cavity voltage is lowered, thus requiring no bias device.
One such prior art accelerating cavity and its power supply method has been described in "High Frequency Accelerating Cavity for Proton Synchrotron", pp. V-19 to V-30, High Energy Accelerating Device Seminar, OHO '89.
FIG. 1 is a schematic diagram of a conventional untuned-type accelerating cavity 3 and its power supply.
In FIG. 1, the accelerating cavity 3 is comprised of accelerating cavity outer conductor 10; accelerating cavity inner conductor 11A, through the inside of which ion beam 60 passes, which inner conductor is disposed to penetrate one of the side walls of the accelerating cavity outer conductor 10; accelerating cavity inner conductor 11B, which is disposed to penetrate the other side wall of the accelerating cavity outer conductor 10; eight toroidal magnetic cores 20, each disposed around the outer surface of the accelerating cavity inner conductor 11A within the accelerating cavity outer conductor 10; and a gap 12 formed between the accelerating cavity inner conductors 11A and 11B. Each side wall at both end portions of the accelerating cavity outer conductor 10 is connected to one of the accelerating cavity inner conductors 11A and 11B. The other ends of the accelerating cavity inner conductors 11A and 11B are connected respectively to a vacuum duct of a circular accelerating device (not shown).
A high-frequency supply of power, which is output from a high-frequency power source 30, is applied across the accelerating cavity inner conductor 11A and the accelerating cavity outer conductor 10, and both conductors in combination constitute a coaxial structure. This power supply method will be referred to as a direct coupling or direct power supply arrangement. By means of this direct power supply arrangement, high-frequency current 41 is caused to flow between the accelerating cavity inner conductor 11A and the accelerating cavity outer conductor 10.
This high frequency current 41 generates a high frequency magnetic field 42. Then, the high frequency magnetic field 42 and the toroidal magnetic cores 20 disposed within the accelerating cavity outer conductor 10 are inductively coupled to generate an accelerating voltage in the gap 12.
By way of example, an accelerating cavity as disclosed in JP-A Laid-Open No. 63-76299 is arranged to supply electric power using the same direct power supply arrangement as in the prior art accelerating cavity 3 of FIG. 1.